Related: panel-plugins/xfce4-whiskermenu-plugin!13

So VERSION_FULL in about dialogs etc. gives e.g. 4.20.0 instead of
4.20.0-UNKNOWN.

Related: thunar!603

To do without xfce4-dev-tools when building from tarballs. This should
probably remain exceptional, so if a replacement can be found, such as
glib-compile-resources for xdt-csource, it should be used instead.

Related: panel-plugins/xfce4-sensors-plugin!76

The removal of exo from the doc and build will take place at a later
date, perhaps only at the end of cycle 4.22.

Related: exo!87

Related: xfwm4!168

Related: xfce4-panel!169

Related: apps/xfce4-terminal!97

Related: panel-plugins/xfce4-clipman-plugin!34

See https://mesonbuild.com/Reference-manual_functions.html#project_license

Fixes: b3503511
Closes: #68

Related: xfce4-session!73

We can't just rely on the `buildtype` option to decide which flags to
add, as the user can directly set the `debug` and `optimization`
options, which usually result in a `buildtype` option set to `custom`.
It also seems that `buildtype=debugoptimized` corresponds to a
production build, for which we shouldn't enable debug logs (see
reference below).

This doesn't exactly reproduce the autotools build mode of the
`--enable-debug` option, which we should probably accept having to
deviate from:
* `--enable-debug={full,yes}` are mixed up in the same debug mode, where
  all debug logs are enabled (which is perhaps no bad thing?). This
  corresponds to `buildtype=debug`.
* `--enable-debug=minimal` corresponds to `buildtype=debugoptimized`,
  where no flag is set.
* `--enable-debug=no` corresponds to `buildtype={minsize,release}`,
  where we set aggressive optimization flags.
* `buildtype=plain` is also a production mode, for which we don't set
  debug flags, but for which we don't really know the optimization mode,
  so we don't set aggressive optimization flags either.

Related: !98
See-also:
https://blogs.gnome.org/mcatanzaro/2022/07/15/best-practices-for-build-options/
https://mesonbuild.com/Builtin-options.html

And give the option of overriding default prioritization with the
BUILD_SYSTEM environment variable. This also fixes the unlikely case
where no valid build system is detected.

Related: xfce4-settings!163

Related: xfce4-settings!163

xfce4-notifyd optionally depends on this, but since we are now using
--auto-features=enabled for the meson CI builds, we need to have this
dep present.

This adds more detailed information about gobject-introspection, as well
as how to build vala API descriptions.  It also includes information
about detecting the presence of library functions, libm, and shipping
the meson build files in the autotools tarball.

See:
https://lists.gnu.org/archive/html/bug-gettext/2024-05/msg00014.html

One of these may be the empty string, if either the autotools build or
meson build is missing.  Passing the variables bare will make the
command line think there's nothing there at all, but quoting an empty
string acts as an empty string command line argument, which 'git diff'
will get confused by.

Usually it's of course a good practice to quote variables that contain
filenames, since they could have spaces in them or other shell-unsafe
characters.  But in this case we know the variable will contain a string
that consists of only letters and dots.

Closes: #65